In electrical apparatus such as power transformers, it is common to construct a magnetic core having cruciform or rectangularly-shaped leg members around which are wrapped a plurality of electrical windings each of which is composed of a plurality of layered conductor turns. Insulating structures of many shapes are used to physically separate the electrical conductors to prevent current transfer between adjacent electrical conductors and other electrically conducting members or parts of the apparatus. In addition, insulating structures are often used to form a void or open space between adjacent electrical conductors to facilitate cooling. These insulating "spacers" allow dielectric fluid to freely flow between the windings forming the transformer. The dielectric fluid therebetween provides additional insulation which is often required when the apparatus is to be operated at high voltages.
The arrangement and composition of the insulating structures which perform this spacing function depends largely upon the structural characteristics of the electrical apparatus and its voltage rating. In almost all electrical apparatus, the insulation structure must not only hold one electrical conductor at a spaced distance from another part or member but must also support one or more electrical conductors relative to the base or foundation upon which the windings are carried. Insulating structures which have mechanical integrity are particularly desirable in power transformers where conductor movement may be caused by excess forces such as those encountered when a transformer is subjected to a short-circuit load. A loose insulation structure not only permits movement of the conductors during a short-circuit condition but also permits movement due to thermal cycling and during shipping due to vibration.
In the case of a two-winding transformer, the low voltage winding is normally disposed adjacent one leg of the magnetic core with the high voltage winding wound around the low voltage winding. During a short-circuit condition, the low voltage and high voltage windings tend to separate, i.e. move in opposite directions. In particular, the low voltage winding is compressed against the leg of the magnetic core while the high voltage winding is subject to an outwardly directed radial force. As can be expected, considerable mechanical force is exerted against the spacers and other insulating structures between the windings. The force may be sufficient to pull the spacer out of position. This causes misalignment of the windings and (assuming the spacers are sufficiently distorted so as to change the distance or gap between the windings) a reduction of the insulated strength provided by the dielectric system.
According to the prior art, insulating structures or spacers are usually formed from solid insulated material such as pressboard of sufficient thickness to form a cooling duct or channel between adjacent windings. These spacer members are normally disposed so as to be flatly in contact with adjacent windings or the core support structure around which the windings are wound. During a short-circuit, there may be sufficient force or relative movement between the two windings such that the spacer is free to move out of alignment. If the spacer is held in place with an adhesive or mechanical connection, the force is often great enough to break the adhesive bonds or mechanical connection and pull the spacer structure against the windings, thereby resulting in misalignment of the spacers and the adjacent windings.
Heretofore, the windings of a transformer have been separated from the magnetic core legs and from adjacent windings by dowels which were wedged in place. These dowels are often arranged around a cruciformed rectangular core so as to form a generally cylindrical base structure. It is upon this structure that the electrical conductors were wound to form the windings of the transformer. The arrangements shown in U.S. Pat. Nos. 4,199,862; 4,173,747; and 3,789,337 are typical. It will readily be apparent by studying the foregoing patents that the problem of spacing apart one or more windings of an electrical apparatus or one or more of the electrical conductors of an electrical apparatus winding, in such a manner that adequate insulation and mechanical strength is provided, is a problem that has not been completely solved. Moreover, it should be clear that there is a long felt need for a solution to the problem of providing an efficient, low cost, mechanically strong spacer especially in view of the failure of so many others.
While dielectric fluid is necessary to provide adequate insulation and to provide adequate cooling, there are many locations within the apparatus where the dielectric fluid is not specifically needed for insulating or cooling purposes. This is particularly true when the tank or container in which the apparatus is housed is rectangular and the electrical apparatus within the tank has an ellipsoidal cross section. Various schemes have been devised to minimize, or reduce, the amount of fluid contained within the tank structure. Fisher (U.S. Pat. No. 3,979,552) recognized that earlier proposals and methods to reduce the amount of oil used in electrical apparatus had new meaning with the ever increasing price of petroleum products. Fisher's teachings were an advancement over those of Montsinger (U.S. Pat. No. 2,036,068). Montsinger suggested the replacement of a portion of the liquid coolant used in a transformer with spheres of fired clay. Fisher proposed a thermal insulating medium comprising a plurality of glass spheres having one or more closed voids. The glass spheres were sufficiently small such that they could occupy most of the available free space within the transformer tank. Glass beads were also proposed by Theodore in U.S. Pat. No. 3,670,276. Galloway (U.S. Pat. No. 3,644,858) taught the use of foam resin blocks to cushion the core-winding assembly of a transformer; in particular, a porous polyurethane resin was disclosed. More recently, Eyestone (U.S. Pat. No. 4,172,965) used a polyurethane encapsulate to provide an array of stacked coils in an inductive assembly.
This latter set of patents is distinguished from the earlier set of patents in that the latter is representative of structures which completely filled the void space between adjacent windings while the earlier set of patents described structures which space apart the windings at discrete locations. In addition, the latter set of patents teach arrangements which provide little, if any, structural support and which have the primary effect of displacing dielectric fluid.
Significantly, those skilled in the art have heretofore neglected the teachings exemplified in this latter set of patents in approaching the problem of designing an insulating support structure for electrical apparatus such as a transformer. There is no suggestion by any of these inventors of a method or apparatus which could combine these latter teachings in an insulating or spacing structure which can be positioned at discrete locations within the interstices of the transformer core structure to provide structural support, without significantly affecting the distribution of dielectric fluid. Discrete positioning of completely solid insulated members, whose width is small relative to the gap between adjacent members, has the effect of producing greater variances in temperature distribution and a less uniform electrostatic field. It would be especially desirable if: the temperature distribution and electrostatic field could be kept as uniform as possible; uniform support could be provided throughout the winding structure; more expensive or costly dielectric fluid could be displaced; and the resulting support structure could accommodate local mechanical distortions and rearrangements brought about by vibration or short circuiting of the windings without disrupting coolant flow or the arrangement of the windings. A device having all of these benefits and advantages, which could be easily adapted to existing transformer designs, and which is relatively inexpensive and easy to install would be widely accepted by the industry.